Forest Fire in the American Southwest
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Mechanical Thinning

There is growing agreement among scientists and other experts on the need for small tree thinning and prescribed burns for fuels reduction. There is no one-size-fits-all prescription, but thinning treatments that focus on small diameter trees and brush lessen the severity of forest fires and facilitate reintroduction of beneficial fires where appropriate.

In the ponderosa pine forests of the American Southwest, approximately 90% of the trees in the forest are smaller than 12 inches in diameter. These small trees comprise the vast majority of fire risk to communities by creating a ladder fuel effect into the overstory canopy. Forests where ladder fuels are limited and tree crowns (or the crowns of groups of trees) are separated won’t support a crown fire.

It is best to maintain structural diversity within and among groups, using crown development, canopy continuity and stem densities as guides. By removing smaller trees from the spaces and openings among groups the potential for sustained crown fire is reduced and the growth of the understory community encouraged.

On the stand level, an entire range of tree sizes and ages should be retained. Older and larger trees are rare after 100 years of logging in the forest. They tend to be fire resistant and should be preserved and protected if they are healthy. A few completely unthinned areas serve as species refugia, wildlife visual cover and travel corridors.

Traditional harvesting methods may not be suitable in the wildland-urban interface, while in other areas it may be the preferred method. The use of smaller equipment reduces soil compaction and disturbance, minimizing the amount of soil exposed for noxious weed and other exotic plant establishment, as well as soil erosion.

Follow-up thinning and maintenance burns are scheduled as necessary to ensure long-term reduction of the risk of destructive fire.


Last edited June 25, 2003


Allen, C.D., M. Savage, D.A. Falk, K.F. Suckling, T.W. Swetnam, T. Schulke, P.B. Stacey, P. Morgan, M. Hoffman, and J. Klingel. 2002. Ecological restoration of southwestern ponderosa pine ecosystems: A broad perspective. Ecological Applications 12(5):1418-1433. Available online at 4/17/03. An important recent paper promoting a broad and flexible perspective on ecological restoration in Southwestern forests.

Ecological Restoration Institute. "What is Ecological Restoration?" 3/31/03. How to apply ecological restoration to Western ponderosa pine ecosystems.

Center for Biological Diversity. Fire and Ecosystem Health. 4/1/03. Outlines the Center’s recommendations on ponderosa pine ecosystem restoration – thinning small diameter trees, leaving large fire-resistant trees, and reintroducing natural fire occurrence.

Greater Flagstaff Forests Partnership. "Ecological Restoration Projects." 4/1/03. Describes several specific restoration projects proposed and underway in the Flagstaff, Arizona area. The site as a whole offers more general information on restoration and the Partnership.

Mast, Joy N. 2003. Tree health and forest structure. Pages 215-232 in Friederici, Peter, ed. 2003. Ecological Restoration of Southwestern Ponderosa Pine Forests : A Sourcebook for Research and Application. Washington, D.C. Island Press, 544 p. How the health of both individual trees and forest stands is affected by growing conditions and various restoration treatments.

Southwest Forest Alliance. 4/1/03. SWFA advocates a “conservative” approach to restoration in the Flagstaff, Arizona area, thinning some small diameter trees while preserving old growth.


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NAU's Program in Community, Culture and Environment Northern Arizona University